Emphasize introduction of WinForms class

Chapter 7: Concluding C# 2: the final features: P195

Listing 7.6 has the WinForms::Button and WebForms::Button in bold, but not the introduction of class WinForms. It's easy to assume that the only changes between listing 7.5 and 7.6 are the changes to alias syntax, at which point it's not clear why you need it. The listing should look like this: